Passive high frequency image reject mixer

ABSTRACT

The apparatus is a complete passive implementation of an image reject mixer (IRM) that is capable of operating at very high frequency. Using a hybrid as part of the IRM circuit enables operation at very high frequencies that also employs a high intermediate frequency (IF). All the components of the design are passive and implementable in MOS technologies providing significant cost and implementation advantages. Furthermore, the apparatus is operative at frequencies that are higher than several tens of GHz.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 61/213,756 filed Jul. 10, 2009.

STATEMENT OF SUPPORT

The work that led to the development of this invention was co-financedby Hellenic Funds and by the European Regional Development Fund (ERDF)under the Hellenic National Strategic Reference Framework (NSRF)2007-2013, according to Contract no. MIKRO2-34 of the project “NextGeneration Millimeter Wave Backhaul Radio-THETA”, within the Program“Hellenic Technology Clusters in Microelectronics-Phase-2 Aid Measure”.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to image reject mixers, and morespecifically to image reject mixers operating at frequencies reachingseveral tens of gigahertz and above.

2. Prior Art

As wireless communication progresses, the frequency of operationincreases dramatically. Currently frequencies of tens of GHz areemployed and are expected to increase over time. Operation at highfrequencies is also known to be demanding in power as well astechnology. That is, for the same design, the higher the frequency ofoperation the higher the power consumption. The higher frequency rangealso requires use of more esoteric manufacturing technologies, such asGaAs, that are capable of effectively addressing the frequencyrequirements, but such manufacturing technologies have a price from botha technology and a power perspective. As long as demand for suchproducts is low, such technologies are tolerated, but as the need formass production arises, it is required to utilize technologies that aremore power friendly as well as less esoteric.

In order to reduce costs, there is a tendency to move from a moreexpensive process technology to a lower cost process technology, e.g.,moving from GaAs to CMOS. However, a lower cost technology, such asCMOS, may suffer from other disadvantages. In the area of image rejectmixers (IRMs) targeted to operate at frequencies in the range of tens ofGHz, CMOS based technologies are not currently used, and hence theadvantages associated with such technologies are not achieved. Moreover,active components are used, with the Gilbert cell being the prominentsolution. An alternative approach is shown in FIG. 1. The MOSdown-conversion multiplier mixer 100 comprises a quad MOS cell 110 and atransresistance amplifier 120 that is used to convert the mixer outputcurrent to voltage signal. Together with the capacitors 130, it furtheracts as a low-pass filter in order for only the intermediate frequency(IF) to pass through. The quad MOS cell 110 is a balanced mixeroperative as a multiplying mixer. The RF signal at the input of themixer is multiplied by the local oscillator (LO) signal. The low-passfilter of this circuit 100 cuts-off the RF+LO product while maintainingthe IF RF−LO frequency.

The current art is limited because of its inability to provide frequencymixers that are low on power consumption, are implemented oncost-effective integrated circuit (IC) technologies, and are capable ofoperating at high intermediate (IF) frequencies, preferably in the GHzrange. It would therefore be advantageous to provide a solution thatovercomes the prior art limitations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a multiplying down-conversion mixer (prior art).

FIG. 2 is a schematic diagram of a passive IRM implemented in accordancewith the principles of the invention.

FIG. 3 is a hybrid element used as an element of the passive IRM.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus of the present invention is a complete passiveimplementation of an image reject mixer (IRM) that is capable ofoperating at very high frequency. Using a hybrid as part of the IRMcircuit enables operation at very high frequencies that also employ ahigh intermediate frequency (IF). All the components of the design arepassive and implementable in MOS technologies providing significant costand implementation advantages. Furthermore, the apparatus is operativeat frequencies that are higher than several tens of GHz.

In accordance with the principles of the invention, the mixer solution,depicted in FIG. 1, is modified as an IRM where the filtering actionimplemented with a transresistance amplifier is replaced by alumped-element hybrid. The hybrid, further described in FIG. 3 below,functions as both a band-pass filter and as the image rejectionapparatus. The result is a fully passive circuit that does not require abias current for its operation. Reference is now made to FIG. 2 thatdepicts a schematic diagram of the passive IRM 200 implemented inaccordance with the principles of the invention. Basically it comprisesquad MOS elements 110 (110-1 and 110-2, also see FIG. 1), typicallyoperative in the deep triode region, a phase shifter 220 and hybrids230. The purpose of the image-reject mixer 200 is to reject the image(IM) signal that is down-converted. Present are a RF signal and a localoscillator (LO) signal. The IM signal is IM=RF+2*IF. Hence, if RF<LO,then IF=LO−RF and IM=RF+2*IF=RF+2*(LO−RF)=2*LO−RF. When it isdown-converted this becomes: 2*LO−RF−LO=LO−RF=IF. In other words, thereis a product from the unwanted frequency IM falling into the IF signaland it is therefore necessary to distinguish the wanted RF from theunwanted IM. The solution is to use the IRM and in accordance with theinvention, to use the hybrid. Accordingly the first output of the hybridcarries the IF component that comes from the wanted RF frequency and thesecond hybrid output carries the IF component coming from the IMfrequency. It is the positioning of the RF signal with respect to the LOsignal that defines which output carries which product. This requiresdetermination whether RF is greater than LO or RF is smaller than LO andthere upon selecting the desired output from the hybrid, as furthershown with respect to FIG. 3.

The passive mixer (passive meaning incapable of or at least notproviding gain) and hybrid combination shown in FIG. 2 not only providesimage rejection but also acts like a passive filter to reject thehigh-frequency component (RF+LO) and eliminates the need for current tovoltage conversion as the hybrids are current-driven directly from thequad MOS transistors.

Notably, the MOS cell 110 of FIG. 2 operates as a multiplying mixer,where the output is a multiplication of the radio frequency (RF) by thelocal oscillator (LO), or in other words RF×LO, resulting in an outputof both RF+LO and RF−LO frequencies. Multipliers 110 hence receive thedifferential RF signals RF+ and RF−. In addition multiplier 110-1receives the differential LO signals LO+ and LO− while the multiplier110-2 receives the same LO signals with a 90° phase shift using phaseshifter 220. The phase shifter 220, according to the invention, is apassive element, and in a typical implementation, a Lange coupler,implemented in a silicon-based technology, may be used. Accordingly,multiplier 110-1 outputs the “I IF+” and “I IF−” differential signalsand multiplier 110-2 outputs the “Q IF+” and “Q IF−” differentialsignals. The outputs from the multipliers 110-1 and 110-2 are providedto lumped-element hybrids 230. A detailed description of alumped-element 230 is provided with respect to FIG. 3 below. The “I IF+”output from multiplier 110-1 and the “Q IF+” output of multiplier 110-2are provided to the inputs of hybrid 230-1. The “I IF−” output ofmultiplier 110-1 and the “Q IF−” output of multiplier 110-2 are providedto the inputs of the hybrid 230-2. The hybrids 230 provide the wanted“IF+” and the wanted “IF−” signals at the outputs of hybrids 230-1 and230-2 respectively, and are further explained below with respect to FIG.3.

To further understand the principles of the invention, reference is madeto FIG. 3, depicting a lumped-element hybrid 230. The hybrid 230comprises four inductors and four capacitors designed to mimic thebehavior of a classical microwave Branchline coupler, without using MMICtechnologies. The “I” input of the hybrid 230 is coupled to one port ofthe capacitor 234-1, the other port of which is connected to ground.Also coupled to the “I” input are two inductors, inductor 232-1 and232-4. The other terminal of inductor 232-1 is coupled to the “Q” inputof hybrid 230 as well as to terminals of capacitor 234-2, the other portof which is connected to ground, and to inductor 232-2. The other portof inductor 232-4 is coupled to the terminal 237 of the hybrid 230 aswell as to terminals of capacitor 234-3, the other port of which isconnected to ground, and inductor 232-3. The other ports of inductors232-2 and 232-3 are coupled to each other as well as to a port ofcapacitor 234-4, the other port of which is coupled to ground, and tothe terminal 238 output of hybrid 230. A switch 236 determines which ofthe signals present at terminals 237 and 238 is the “wanted IF” signaland accordingly connects that terminal to the IF_(O) output of thehybrid 230. The switch has a first position when RF>LO and a secondposition when RF<LO. In one embodiment of the invention, one or more ofthe capacitors 234 are tunable, thereby allowing tuning of the hybrid.For this purpose, a switched capacitor bank (not shown) or a variablecapacitor (not shown) may be used, both of which are well known in theprior art, resulting in a variable hybrid 230. In an exemplaryapplication with an IF above 1 gigahertz, such as an IF in the range of1-7 GHz, the inductor range is 1-6 nH and the capacitor range is 1-6 pF.Inductors and capacitors in these ranges are readily and reasonablyimplementable in common manufacturing technologies such as CMOStechnology, thereby allowing for cost-effective implementation of suchintegrated circuits (ICs). Furthermore, using the hybrid 230 at theoutput of the IRM 200 removes the need of having an operationalamplifier at the output of the mixer as required by prior art solutions.In another embodiment, the IF is above 5 gigahertz.

In one embodiment of the invention (FIG. 2), a set of switches (notshown) may be connected immediately at the hybrid output enabling aselection of only one of the two outputs each time. The hybrid 230-1provides the final IF+ and the hybrid 230-2 provides the final IF−. Thereason for using both terminals 237 and 238 of the hybrid, or havingthem available in the first place, is that it allows different treatmentof the output signal when the RF frequency is greater than the LOfrequency than when the LO frequency is greater than the RF frequency.By having the switches at the hybrid outputs, the appropriate outputterminal can be selected depending on the frequency plan of operation ofthe receiver chain. This is of course unnecessary if the frequencies arefixed for a given system, as in such a case one of the two outputs canbe used without employing switches. The other output may be simplyconnected to ground through a 50 Ohm resistor. In yet anotherembodiment, the “I” and “Q” outputs of the two IF signals may be addedto convert the differential output to one single IF output having adouble amplitude. The appropriate hybrid output has the image (IM)component suppressed and this is the image-reject operation of themixer. In accordance with the invention, the hybrids 230 have a doublerole: they operate as a band-pass filter to reject the RF+LO mixeroutput component and as an IM rejection component.

The complete passive implementation of the IRM provides significantadvantage for low power implementations and especially the use ofpervasive CMOS manufacturing technologies. Specifically, it allows animplementation of the IRM for very high frequencies, for example, a 90nanometer CMOS technology allows for a 60 GHz IRM, otherwise notimplementable in such a technology. The use of the hybrid 230 enableshigh-frequency IF, in the range of a few GHz, a range which iscompatible with an RF frequency as high as 60 GHz. Using prior artsolutions, the combination of an active mixer with a hybrid would not beoperative for such a high RF. Vice versa, the combination of a passivemixer with an operational amplifier instead of the hybrid, is notsuitable for such an IF range. Hence the invention overcomes thedeficiencies of the prior art. By further using a Lange coupler toimplement the phase shifter in combination with the MOS switches and thehybrids, all for an all in a Silicon-based process, without compromisingthe performance necessary for a mixer operative in an RF range ofseveral tens of GHz and an IF in the range of several GHz, andimplementing the circuit on an IC.

While certain preferred embodiments of the present invention have beendisclosed and described herein for purposes of illustration and not forpurposes of limitation, it will be understood by those skilled in theart that various changes in form and detail may be made therein withoutdeparting from the spirit and scope of the invention. Specifically, thecircuit disclosed is applicable to any Silicon-based manufacturingprocess, including MOS, BiCMOS and CMOS processes. The upper limit ofthe RF is determined by the process geometry.

1. A passive image reject mixer (IRM) comprising: a first MOS multiplierenabled to receive RF+ and RF− input signals forming differential radiofrequency input signal and LO+ and LO− input signals forming adifferential local oscillator input signal, and further enabled tooutput an intermediate frequency I IF+ and I IF− differential outputsignal; a second MOS multiplier; a passive phase shifter coupled to saiddifferential local oscillator input signal and enabled to output to saidsecond MOS multiplier said differential local oscillator input signalphase shifted ninety degrees; the second MOS multiplier enabled toreceive said differential radio frequency input signal and the output ofthe passive phase shifter, and further enabled to output an intermediatefrequency (IF) Q IF+ and Q IF− differential output signal; a firstlumped-element hybrid coupled to receive said I IF+ output signal ofsaid first MOS multiplier and said Q IF+ output signal of said secondMOS multiplier, said first lumped-element hybrid being enabled to outputan IF+ output signal responsive to whether a frequency of said radiofrequency input signal is greater or less than a frequency of said firstdifferential local oscillator input signal; and a second lumped-elementhybrid coupled to receive said I IF− output signal of said first MOSmultiplier and said Q IF− output signal of said second MOS multiplier,said second lumped-element enabled to output an IF− output signalresponsive to whether a frequency of said radio frequency input signalis greater or less than a frequency of said first differential localoscillator input signal; said first MOS multiplier and said second MOSmultiplier both being operative in deep triode region.
 2. The passiveIRM of claim 1, wherein said IRM is operative with an intermediatefrequency above one gigahertz.
 3. The passive IRM of claim 1, whereinsaid IRM is operative with an intermediate frequency above fivegigahertz.
 4. The passive IRM of claim 1, wherein each saidlumped-element hybrid comprises: a first capacitor, having a firstterminal coupled to an I input of said lumped-element hybrid; a secondcapacitor, having a first terminal coupled to a Q input of saidlumped-element hybrid; a third capacitor, having a first terminalcoupled to a first input of a two-to-one switch of said lumped-elementhybrid; a fourth capacitor, having a first terminal coupled to a secondinput of said two-to-one switch of said lumped-element hybrid; a firstinductor coupled between said I input and said first input of saidtwo-to-one switch; a second inductor coupled between said Q input andsaid second input of said two-to-one switch; a third inductor coupledbetween said I input and said Q input; a fourth inductor coupled betweensaid first input of said two-to-one switch and said second input of saidtwo-to-one switch; and said two-to-one switch being coupled to an IFoutput terminal of said lumped-element hybrid and enabled to selectbetween said first input of said two-to-one switch and said second inputof said two-to-one switch responsive to whether a frequency of saiddifferential radio frequency input signal is greater than or lower thana frequency of the differential local oscillator input signal.
 5. Thepassive IRM of claim 4, wherein at least one of said first capacitor,said second capacitor, said third capacitor and said fourth capacitor isenabled to be tuned to provide variable capacitance.
 6. The passive IRMof claim 4, wherein at least one of said first capacitor, said secondcapacitor, said third capacitor and said fourth capacitor is a switchedcapacitor bank comprising a plurality of capacitors.
 7. The passive IRMof claim 4, wherein in each said lumped-hybrid, each of said firstcapacitor, said second capacitor, said third capacitor and said fourthcapacitor has a capacitance in the range of one to six pico-Farads. 8.The passive IRM of claim 4, wherein in each said lumped-hybrid, each ofsaid first inductor, said second inductor, said third inductor and saidfourth inductor has an inductance in the range of one to six nano-Henry.9. The passive IRM of claim 1, wherein in each said lumped-hybrid, eachsaid passive phase shifter is a Lange coupler.
 10. The passive IRM ofclaim 1, wherein the passive IRM is manufactured using one of: MOSprocessing technology, CMOS processing technology, BiCMOS processtechnology.
 11. A passive image reject mixer (IRM) comprising: adifferential radio frequency input signal having RF+ and RF− inputsignals; a first differential local oscillator input signal having LO+and LO− input signals; a second differential local oscillator signalphase shifted with respect of said first differential local oscillatorinput signal, the second differential local oscillator signal providingphase shifted LO+ and LO− signals; a first passive multiplier responsiveto said RF+ and said RF− input signals, said LO+ and said LO− inputsignals, and outputting I IF+ and an “I IF− output signals; a secondpassive multiplier responsive to said RF+ and said RF− input signals,said phase shifted LO+ and said phase shifted LO− signals, andoutputting a Q IF+ and Q IF− output signals; a first passive filter forfiltering said I IF+ and said Q IF+ output signals and generating an IF+output signal; and a second passive filter filtering said I IF− and saidQ IF− signals and generating an IF− output signal.
 12. The passive IRMof claim 11, further comprised of a passive phase shifter coupled toreceive said first differential local oscillator input signal andprovide said second differential local oscillator signal.
 13. Thepassive IRM of claim 12, wherein said passive phase shifter is a Langecoupler.
 14. The passive IRM of claim 11, wherein said phase shiftershifts said phase ninety degrees.
 15. The passive IRM of claim 11,wherein said first passive multiplier comprises four MOS transistorswherein gates of two of said four MOS transistor are coupled to said LO+signal and gates of the other two of said four MOS transistor arecoupled to said LO− signal, and wherein said second passive multipliercomprises four MOS transistors wherein gates of two of said four MOStransistor are coupled to said phase shifted LO+ signal and gates of theother two of said four MOS transistor are coupled to said phase shiftedLO− signal.
 16. The passive IRM of claim 11, wherein each said passivefilter comprises a lumped-element hybrid.
 17. The passive IRM of claim16, wherein each said lumped-element hybrid comprises: a firstcapacitor, having a first terminal coupled to an I input of saidlumped-element hybrid; a second capacitor, having a first terminalcoupled to a Q input of said lumped-element hybrid; a third capacitor,having a first terminal coupled to a first input of a two-to-one switchof said lumped-element hybrid; a fourth capacitor, having a firstterminal coupled to a second input of a two-to-one switch of saidlumped-element hybrid; a first inductor coupled between said I input andsaid first input of a two-to-one switch; a second inductor coupledbetween said Q input and said second input of a two-to-one switch; athird inductor coupled between said I input and said Q input; and afourth inductor coupled between said first input of said two-to-oneswitch and said second input of said two-to-one switch; an output ofeach said two-to-one switch being coupled to a respective IF outputterminal of said lumped-element hybrid and enabled to select betweensaid first input of the respective said two-to-one switch and saidsecond input of the respective said two-to-one switch responsive towhether a frequency of said radio frequency input signal is greater orless than a frequency of said first differential local oscillator inputsignal.
 18. The passive IRM of claim 17, wherein in each saidlumped-element hybrid, at least one of said first capacitor, said secondcapacitor, said third capacitor and said fourth capacitor of each saidis enabled to be tuned to provide variable capacitance.
 19. The passiveIRM of claim 17 wherein in each said lumped-element hybrid, at least oneof said first capacitor, said second capacitor, said third capacitor andsaid fourth capacitor is a switched capacitor bank comprising aplurality of capacitors.
 20. The passive IRM of claim 17, wherein ineach said lumped-element hybrid, each of said first capacitor, saidsecond capacitor, said third capacitor and said fourth capacitor has acapacitance in the range of one to six pico-Farads.
 21. The passive IRMof claim 17, wherein in each said lumped-element hybrid, each of saidfirst inductor, said second inductor, said third inductor and saidfourth inductor has an inductance in the range of one to six nano-Henry.22. The passive IRM of claim 11, wherein a frequency of said IF+ and IF−output signals is at least one gigahertz.
 23. The passive IRM of claim11, wherein a frequency of said IF+ and IF− output signals is at leastfive gigahertz.